Maroon Divider
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If soils have insufficient capacity to supply plant nutrients to support crop production, fertilizers or other soil amendments will be required. Although some fertilizers may be applied as foliar sprays, the principal use of fertilizers is to provide plant nutrients directly to the soil. Soil amendments such as liming alter soil conditions so that soil-borne nutrients may become available to plants.

Plant Nutrients

Sixteen chemical elements are known to be essential for plant growth (Table 16). These elements are plant nutrients. For an element to be considered as a plant nutrient, it must meet all of several criteria.

1. The element must be required for a plant to complete its life cycle.

2. The role of the element in plant growth must be specific and direct and not replaceable by another element.

3. The requirement for the element must be general among plants.

Table 16. List of elements essential for plant growth.

Supplied by air

or water

---------------------------Supplied by soil-------------------------------------






















Several elements such as, sodium, vanadium, silicon, selenium, cobalt, and others have some of the characteristics of essential elements, but do not meet all of the criteria for essentiality. Specific, direct roles of these elements in plant metabolism have not been defined, or their requirements have not been demonstrated to be general among plants. Elements that enhance yields or improve growth of some plants, but which are not required absolutely by the plants, are called beneficial elements.

Nitrogen is a good example of an essential element, for it is a constituent of protein and other vital compounds in plants. Without a constant and abundant supply of nitrogen plants will not grow and will exhibit characteristic symptoms of deficiency (Table 17). No other element will substitute for nitrogen, and all plants require nitrogen. On the other hand, cobalt is required only by nitrogen-fixing plants, such as legumes. Legumes do not absolutely require cobalt to grow and to complete their life cycle, for they will grow well if supplied with nitrate or ammonium. Utilization of nitrate or ammonium does not require cobalt. Cobalt is a beneficial element.

On a dry weight basis, plants are mostly carbon, oxygen, and hydrogen, being about 50%, 40%, and 5%, of these respective elements. Plants receive carbon, oxygen, and hydrogen from air and water. The other sixteen elements are obtained primarily from the soil. They are classed as macronutrients or micronutrients on the basis of the relative amounts that plants accumulate and need for their growth. The macronutrients are accumulated in plant tissues in concentrations ranging up to several percent on a dry weight basis, whereas the micronutrients are accumulated in amounts as low as one part per million (ppm) and up to a few hundred ppm (hundredths of percent). The primary macronutrients, nitrogen, phosphorus, and potassium, are deficient in more than half of the soils that are cropped in the United States. Deficiencies of the remaining essential elements are more rare and are endemic with regional soils, particular crops, or unbalanced conditions of plant nutrition.


Fertilization after diagnosis of deficiency

In some cases, if the deficiencies are identifed during the growing seasons, the symptoms of deficiency may be diagnosed, and corrective action can be taken by fertilization. Restoration of growth by fertilization is likely, but not ensured, if the symptoms have not progressed so far as to have irreversibly damaged the plants. Plants normally respond quickly to nitrogen fertilization. Response to the other elements in fertilizers may be slow. Young plants are more likely to overcome deficiencies after fertilization than old plants. Generally, any time that a deficiency occurs during the growth of plants, some of their yield potential or other qualities are lost for that season. Diagnoses of nutrient deficiencies often are most valuable as guidelines for fertilization of crops in the next growing season.

Table 17. Symptoms of deficiency of plant nutrients.


 Nutrient Symptoms of deficiency _______________________________________________________________________________________________

Nitrogen Plants are stunted and spindly. Foliage is light green or yellow. Lower leaves become yellow and drop off.

Phosphorus Plants are stunted but may be dark green. Lower leaves may redden particularly on the under sides and become progressively yellow and necrotic.

Potassium Lower leaves will be necrotic, scorched, along the edges and tip. Stems will be weak, and plants may fall over, lodge. Fruits may be malformed, and seeds may be unfilled, chaffy.

Calcium The growing points and immature leaves will die back. Vegetables derived from buds or heads may have internal "rot" or tipburn. Fruits may show blossom-end rot or have short storage life.

Magnesium Leaves are mottled with interveinal yellowing. Symptoms start on lower leaves. A few species show purpling of foliage.

Sulfur Plants are stunted and spindly. Foliage is light green or yellow with the symptoms appearing first on the young leaves.

Iron Young leaves appear yellow or bleached white, often with mottling or striping. With severe deficiency, symptoms progress to older leaves.

Zinc Young leaves show interveinal yellowing. Shoots may die back. Rosetting may occur.

Copper Young leaves may show white spots, bleaching, or dieback.

Manganese Young leaves may show interveinal yellowing or dieback. Lower leaves of some species may show dead or gray spots.

Molybdenum Old and middle leaves become yellow. Edges of leaves may roll, die, and flake off.

Boron Growing points die. Crowns rot. Flowers are abnormal (e.g., brown curd of cauliflower).

Chlorine Young leaves develop off-color bronzing, yellowing, or necrosis. Chlorine deficiency has never been detected in nature.

Nickel Failure of seeds to germinate. Chlorosis of young leaves. Necrosis of meristems. Deficiency not detected in soils.


Diagnoses of deficiencies are most accurate if the symptoms are noted in their early stages of development. In advanced stages, distinction between symptoms of deficiency of one element and another is difficult. The more severe the injury, the more the deficiency symptom of one element resembles the deficiency symptom of another. Sometimes stresses, such as drought, heat, overwatering, or mechanical injury, cause development of symptoms that are mistaken for nutrient deficiencies. This kind of misdiagnosis is common in assessing stresses on house plants and is not uncommon in the field or garden.

To avoid confusing one symptom of stress with another, accurate records or recollections are needed. If a deficiency is noted at its inception, if progression of the symptom is recorded, and if records of management and fertilization are kept, fertilization according to diagnosis of nutrient deficiencies can be successful. In this sense, a growing plant is one of the best soil test kits available.

In practices in which management by the grower is high, fertilization may be in accordance with tissue testing. Interpretation of tissue tests often requires the advise of consultants or researchers in plant nutrition. Tissue testing may be helpful in identifying or confirming diagnoses by comparing results of the tests with standards developed from tests of plants on adequate nutrition. Tissue testing is applied also to interpret the nutritive status of a plant at a given stage of development by comparing test results with those obtained by research with plants on adequate nutrition at the same stage of development. With tissue testing, recommendations for fertilization can be made for nutrition of a crop in the current season.

Fertilization in accordance with soil testing

The establishment and maintenance of soil fertility often involves soil testing. Fertilization in accordance with the results of a soil tests is much more common than fertilization as the result of tissue testing or plant analyses. Soil testing has particular value in establishment of a program of soil fertility. In newly cropped land or in cases in which the history of cropping and managment are not known, soil testing can provide a foundation for building a program of soil fertility. In cases in which the history of cropping and management are known, fertilization in conjunction with experience and previous productivity of crops may be more valuable than soil testing. From soil testing, attempts are made to assess the amount of plant nutrients that will be available to support plant growth and then to develop recommendations for fertilization. A reliable soil test must have three sound components:

1. The sample must be representative of the area to be fertilized.

2. The analytical procedure must be accurate for estimating availability of nutrients.

3. The results must be calibrated with plant responses in the field.

Sampling. The soil sample must be taken properly; otherwise, the analytical and calibration steps are placed in jeopardy. Unless the sample is truly representative, other matters, such as differences in analytical procedures and philosophies about fertilization, become unimportant. Parameters of soil fertility as reflected by a soil test can vary from site to site at a given location and with season.

Selection of a one best time for sampling for testing of all nutrients cannot be done. However, if a general preference is to be given, for fertilization in the spring, sampling in the fall is better than sampling in the spring. Short-term variation in tests for available nutrients is less in the fall than in the spring. Soil pH may be about 0.5 unit lower in the fall than in the spring and may be a better indication of soil acidity than measurements made in the spring after a period of biological inactivity over the winter. Taking soil samples during the growing season and after crops have been fertilized is a poor practice. Variation with time and with location will be large in fertilized fields, and collection of a representative sample will be difficult.

The number of samples to be taken depends largely on the variability that exists in the land at a site. The size of the plot is of lesser importance that the on-site variability within a plot. In general, the more samples that are taken, the more representatively a plot can be sampled. However, if a plot is uniform, only one sample needs to be taken, although this one sample may be a composite of several subsamples (Figure 11). The unit area for sampling may be as large as 10 acres or more, that is, for plots of less than about 10 acres, only one sample is needed. Therefore, from a garden or yard, only one sample usually is necessary. If it is determined that a lot of on-site variablity occurs, a sample should be taken from each uniform area (Figures 11 & 12). For example, if part of a field is limed and part is unlimed and these areas are dilineated, one sample should be taken from the limed area and one should be taken from the unlimed area. Sloping and flat areas should be sampled separately. Areas with green manures turned under should be sampled separately from unmanured areas. Areas in which problems are noted that are different from other areas should be sampled separately. Common sense and economics play roles in determining the number of samples to be collected. Costs of analysis are fairly expensive, so the number of soil tests should not go beyond practical, economical limits.

Collection of samples from a site may follow several patterns. Generally, a single sample is not taken, but several are taken in a random or precise pattern and combined to make one composite sample. Random sampling generally gives more variability in test results than collecting of samples in a pattern. Often, single samples are collected in a pattern by selecting a central location and taking a sample at that location and at others at the corner of a rectangle a few paces from the center then combining all samples into one (Figure 11).

Figure 11. Design of site for collection of a single soil sample for testing.


If it is determined that a lot of on-site variability, a sample should be taken from each uniform area (Figure 12). For example, if part of a field is limed and part is unlimed and these areas are delineated, one sample should be taken from the limed area and sone should be taken from the unlimed area. Sloping and flat areas should be sampled separately. Areas with green manures turned under should be sampled separately from unmanured areas. Areas in which problems are noted that are different from other areas should be sampled separately. Common sense and economics play roles in determining the number of samples to be collected. Costs of analysis are fairly expensive, so the number of soil tests should not go beyond practical, economic limits.






Figure 12. Maps for collection of soil samples from (A) Uniform plots of land and nonuniform plots with (B) Lime applied or not applied, (C) Non uniform terrain, and (D) Nonuniform productivity.


Other systems of sampling large areas are the zigzag pattern and strip pattern (Figure 13). In the zig-zag and strip systems, individual samples are kept separate. Sampling at individual sites may follow the pattern in Figure 11 for taking individual samples. Zigzag sampling gives an estimate of the mean fertility of the plot, and the recommendation would include a uniform fertilization of the area. Strip sampling reveals pattern of variability, and fertilizer recommendations can be modified according to that variability.


Figure 13. Map for collecting of soil samples by (A) Strip Sampling or by (B) Zig-zag Sampling. Solid lines indicate paths for walking. Individual samples (X) are collected as indicated in Figure 11.


Taking the soil sample can be accomplished easily with a shovel or trowel. The soil should be dug to about 6 inches (Figure 14). Soil from the vertical cut can be shaved off to create a portion of sample. About a pint of sample should be taken in total. The sample should be air-dried on waxed paper or plastic. After the sample is dry, stones and large fragments of organic matter should be picked or sieved out, and the sample should packaged in a glass, waxed paper, or plastic container. Soil testing laboratories often specify the kind of packaging and handling that a sample should receive before delivery to the laboratory. If the sample is not be tested in a week or two, it should be stored in a freezer.


Figure 14. Illustration of procedure for collecting soil sample.

Analytical procedures. Soil tests are conducted most commonly by state soil testing laboratories. Commercial laboratories also provide this service. Most agencies charge for the testing, and costs will vary depending on the number of factors that are measured in the tests. Soil testing kits can be purchased at garden and farm supply stores. If several samples, say 10 or more, are to be tested, these kits may be economical. On the other hand, if only a sample or two are to be tested, the laboratories will provide the more economical service.

All testing procedures involve rapid or quick tests that are based on analysis of the soil by quantitative or semiquantitative methods. Most soil tests assess fertility with respect to nitrogen, phosphorus, potassium, and acidity. Some tests by laboratories or by kits include calcium and magnesium. Some soil testing laboratories and kits estimate availability of some micronutrients and some nonessential elements, such as aluminum, lead, or cadmium.

Soil testing laboratories provide reliable analyses, and if the user works skillfully, reliable results also can be obtained with the do-it-yourself kits. Therefore, analytical procedures are not a major weakness in soil testing. Discrepancies in recommendations based on the analytical results arise from differences in philosophies among people who make the recommendations or manufacture the kits. Discrepancies also result from the lack of good calibration of the soil test to performance of plants in response to recommended practices of fertilization in specific fields.

Calibration. Calibration refers to the research that is performed to interpret soil tests and to correlate crop growth or yield responses to recommendations based on soil tests. Calibration is basic to a good testing program. Without an calibration of crop responses to soil levels of plant nutrients and to recommended fertilization, soil tests have little meaning. A properly calibrated soil test provides information on the sufficiency of a nutrient and identifies how much of the nutrient should be supplied in fertilizer. Calibration is a continuing process as more research is conducted on sampling procedures, analytical procedures, and fertilization techniques in the field.

Nitrogen. Soil tests for sufficiency of nitrogen and amounts of nitrogen fertilizer to be supplied are not well calibrated. Most rapid soil tests for nitrogen are based on analysis of nitrate. Although nitrate in the soil can be analyzed precisely and accurately, its determination may not give a true assessment of the fertility of the soil with respect to nitrogen except with certain crops under certain conditions. Nitrate levels in soil fluctuate with rainfall, time of sampling, biological activity in the soil, and storage of samples. Nitrate tests are valuable in dry land agriculture and in irrigated areas in dry regions, where where leaching is minimal. Currently, nitrate tests are being researched and developed specifically for the fertilization of corn fields. Ammonium is measured sometimes in soil testing, but this determination does not improve the calibration. Most of the nitrogen in the soil is in the organic matter. Tests to assess the rate of mineralization of nitrogen in the laboratory are difficult to perform and do not give a good indication of mineralization in the field. Practices for fertilization with nitrogen are best when based on experience or research with a particular plot of land and with the crop that is to be grown.

Phosphorus and potassium. Generally, determinations of available phosphorus and potassium in the soil are better than those of nitrogen, and calibrations of tests for phosphorus and potassium are more reliable than those for nitrogen. Tests for phosphorus and potassium, however, may more accurately predict when not to apply these nutrients, because a sufficient supply is available, than how much to apply.

Other nutrients. Soil tests for calcium, magnesium, sulfur, and minor elements usually do not lead to recommendations as to how much of these elements to apply. The tests reveal adequacies or imbalances in these nutrients and assist in the diagnoses of apparent problems in soil fertility. For example, tests that show low available calcium or magnesium usually are correlated with acid soil conditions and indicate that liming is needed.

Nonessential elements. Soil tests for nonessential elements such as lead, cadmium, and aluminum are run routinely by some laboratories that have capabilities to determine these elements. These tests may assist in the interpretation of problems in soil fertility or assessment of the level of pollution of soil from application of sewage sludge or other materials that may have heavy metals. A high aluminum concentration is an indication of acid soils.

Acidity. Soil acidity is measured by pH or lime tests. This measurement is the most valuable one in soil testing and should be conducted about every 3 years. In soil testing, results of tests for acidity are reported as pH, buffer pH, or lime requirement. pH is an expression of acidity on a logarithmic scale. A pH of 7 is called neutral, for that would be the pH of pure (100%) water. Soils are acid if below pH 7 and alkaline or basic if above pH 7. Mineral soils commonly range in pH from about 4 to 9. Desert soils may exceed pH 9, and organic soils may be as low as pH 3 to 3.5. Regular measurements of pH are made in extracts made with water or with very dilute salt solutions. Regular pH is gives an estimate of the acidity in the soil solution, which is referred to as active acidity. Buffer pH refers to the measurement of pH in a specific kind of extract that is resistant or buffered to changes in pH. Buffer pH is used to calculate the lime requirement of a soil, that is, the amount of lime that is needed to adjust the pH to a given value, for example, pH 6.5 or pH 7. Lime requirement takes into consideration the amount of material needed to adjust the acidity in the soil water (active acidity) and acidity on the soil colloids (reserve acidity). Lime requirement is a function of the pH, texture, and organic matter content of a soil. A properly limed soil is one for which the active acidity and the reserve acidity have been neutralized.

The acidity of soil should be between pH 6 and 7. In this range of pH, conditions are favorable for maximum availability of plant nutrients, and toxicities of soil-borne elements are minimized. Frequently, nutritional deficiencies can be corrected by adjusting the soil pH to the proper range without addition of fertilizers. Above the pH range of 6 to 7, the availability of phosphorus and most minor elements declines because of their reduced solubilities under alkaline conditions. Below this range, solubilities of phosphorus and molybdenum decline. Also, in acid soils, concentrations of aluminum and manganese, which are native in the soil, may increase to toxic levels. A pH range from 6 to 7 is favorable also for microbiological activities, such as those necessary to mineralize organic matter and to oxidize ammonium to nitrate. Availabilities of calcium and magnesium are well correlated with soil pH, for the buffering of soil pH in the range of 6 to 7 is frequently by carbonates of calcium and magnesium. Much more on the practices and effects of liming are covered in Chapter 7.


Maroon Divider
Description | Syllabus | Notes |Guide | Internet | Lab Manual|Exams and Quizzes|Results|More|
Maroon Divider

Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 6, 1999